The results from this study in 4 patients maintained on a constant dosage regimen of phenytoin and carbamazepine indicate that, following oral administration, F is rapidly absorbed from G.I. tract and reaches a mean maximum peak level (Cmax) in plasma of about 1.14 mg/liter in an average time (tmax) of 1 hr. Following the peak the disposition of F was biexponential in all cases. The distribution and elimination half-lives in these patients average 2.07 and 10.06 hr., respectively. The amount of unchanged F recovered in the urine over a 3-day period ranged between 4-15%. Significant contribution of metabolic clearance to total clearance suggests that disposition of F in liver dysfunction may need to be evaluated. The understanding of the disposition of F will help in adjusting its dosage in these patients.